1. Field of the Invention
The present invention relates to an X-ray imaging apparatus and an X-ray imaging method capable of correcting a variation in input/output characteristics of each pixel used in X-ray imaging.
2. Description of the Related Art
In recent years, as medical X-ray imaging apparatuses, digital X-ray imaging apparatuses employing various methods have been becoming widespread, with the advancement of the digital technology. For example, one technique in current practical use is the method of using an X-ray detector in which a phosphor and a large-area amorphous silicon (a-Si) sensor are in close contact with each other to enable a direct digital conversion of an X-ray image without, for example, an optical system. Another technique in current practical use is the method of using, for example, amorphous selenium (a-Se) to directly photoelectrically convert an X-ray into electrons which are detected by a large area amorphous silicon sensor.
In X-ray imaging apparatuses with use of the above-mentioned X-ray detectors, it is generally practiced to correct a variation in sensitivities in every photoelectric conversion element and a difference among input/output characteristics (relationships between an incident X-ray dose and an output value) in every pixel due to, for example, a variation in gains in readout circuits.
For example, if input/output characteristics of individual pixels are guaranteed to be linear, corrections thereof can be achieved by acquiring correction data by projecting an X-ray of a predetermined intensity to the whole surface of a detector without a subject, and dividing a captured image of a subject by this correction data (or subtracting the correction data from the image of the subject after a logarithmic conversion).
However, the input/output characteristic of a standard X-ray detector is known to show nonlinearity when a large incident X-ray dose is projected thereto, since the linearity cannot be maintained due to a saturation phenomenon. Therefore, in such a case, the above-described correction method based on an assumption of linearity cannot provide an appropriate correction to a nonlinear dose region. With the aim of solving this problem, Japanese Patent Application Laid-Open No. 01-235484 discusses a method for correcting such a nonlinear characteristic.
According to this method, a nonlinear characteristic can be appropriately corrected by storing parameters in advance in which the input/output characteristic of each pixel is approximated by a predetermined model function, and using an inverse function of the model function.
However, the method discussed in Japanese Patent Application Laid-Open No. 01-235484 takes into consideration a dose region where the input/output characteristic of each pixel becomes nonlinear, but fails to address a dose region where an output is saturated, i.e., a dose region where an incident X-ray dose that enters a pixel exceeds the acceptable charging amount of that pixel so that the pixel outputs a substantially constant value (hereinafter, this value is referred to as “saturation level”). Once an output reaches the saturation level, a significant change does not occur in the output value relative to an incident X-ray dose to a pixel, so that it is difficult to detect a value of incident X-ray dose from the output value of the pixel. Therefore, the input/output characteristic thereof may not be able to be corrected appropriately.
More specifically, in a dose region where a pixel outputs a substantially constant value, the inverse function cannot produce a unique solution, whereby an appropriate correction is impossible. Further, when the input/output characteristic of each pixel is approximated by a predetermined model function, inclusion of data indicating an output value that reaches the saturation level lowers the approximation accuracy, as a result of which an appropriate correction may be impossible.
On the other hand, it is possible to limit a maximum incident X-ray dose so as to prevent an output value from reaching the saturation level. However, each pixel has a different dose region where the output is saturated, and therefore there are more than a few pixels which the output value reaches the saturation level with a smaller dose compared to other pixels. Then, a problem arises in that, if the maximum incident X-ray dose is limited within such a range that all pixels are prevented from reaching their saturation levels, this results in narrowing of a photographable dose region (dynamic range).